The present disclosure is directed to combustion systems that utilize solid fuel burners. In particular, the present disclosure is directed to boiler and furnace systems that utilize oxygen injection to enrich combustion.
While systems which comprise the combustion of low volatile pulverized solid fuel are generally known, many such systems suffer from the drawback that expensive support fuel is typically required to maintain combustion stability. One approach to solving this drawback is to introduce an oxidant whose oxygen concentration is substantially greater than that found in air (˜21% by volume).
In such “oxygen-enriched” solid fuel applications, particularly those involving low volatile fuels, burner flame stability is strongly dependent upon having the correct minimum amount of oxygen introduced into the Near Burner Zone (NBZ) of the flame. Managing this process is difficult and complex for multiple burner systems. One approach has been to introduce a comfortable excess of burner oxygen above the minimum requirement. An excess amount of oxygen in the burner system suffers from the drawbacks that the operational costs are higher and additional NOx is produced.
Known air-fuel combustion control systems generally infer conditions within the combustion zone by monitoring properties of the combustion product gas stream at one or more locations downstream of the individual flames. However, such known systems are not capable of providing a reliable link between downstream gas conditions and local conditions at a particular burner. Hence, such systems are generally not suitable for individually controlling and/or optimizing combustion at each of the individual burners to reduce undesirable emissions and maintain combustion stability.
Other so-called air-fuel burner management systems monitor radiant emissions of the individual flames for the purpose of detecting a “loss-of-flame” condition. When such a condition is detected, the control system response is to terminate fuel flow to the faulty burner and, in direct-fired systems, other burners supplied by the same pulverizer. Systems of this type are not suitable for converting a measured property into an incremental control action that can continuously adjust combustion conditions pertaining to the burner in question.
The introduction of oxygen into a combustion system, while potentially affording numerous performance and emissions benefits, also introduces a new set of control system challenges that are beyond the scope of the aforementioned prior art control systems. Moreover, if not adequately controlled, the misappropriation of oxygen flows can lead to serious economic, operability and emissions penalties, offsetting any potential benefits.
What is therefore needed is a method and system of controlling oxygen injection into a solid-fuel fired burner and furnace that links burner oxygen injection rates with local flame properties. What is also needed is a method and system of optimizing the distribution of oxygen injection rates among a plurality of locations within a multi-burner boiler or furnace.